SELECT 
  thread_id, 
  type 
FROM 
  cscart_discussion 
WHERE 
  object_id = 5 
  AND object_type = 'M' 
  AND type IN ('B', 'R')

Query time 0.00012

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}